In the intake manifold of an internal combustion engine, an internal passage is largely curved into a substantially C-shape, for instance, as recited in Patent Literature 1, so as to ensure a predetermined path length with which an inertial supercharging effect can be obtained while reducing a size of the intake manifold. Therefore, a lower surface side of the intake manifold which is located on a lower side in an assembled state of the intake manifold relative to an engine body may be formed into a curved shape.